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Chicken Eggs

History

Eggs from many species of fowl have doubtless been consumed since the very beginning of humankind’s stay on earth. In historical times, ancient Romans ate peafowl eggs, and the Chinese were fond of pigeon eggs. Ostrich eggs have been eaten since the days of the Phoenicians, whereas quail eggs, as hard-cooked, shelf-stable, packaged products, are now featured on many gourmet food counters in the United States and Japan. Other eggs consumed by various ethnic groups include those from plovers, partridges, gulls, turkeys, pelicans, ducks, and geese. Turtle eggs have been highly prized, and in starvation situations, any eggs, even those of alligators, have been relied upon.

In this chapter, however, only avian eggs (and these mostly from the chicken) are discussed. Avian eggs in themselves constitute a huge subject: In 1949, A. L. Romanoff and A. J. Romanoff published a book in which they attempted to compile all the facts known, at the time, about the eggs of birds. It contained over 2,400 reference citations.

It is almost obligatory in writing about eggs to first deal with that age-old question: Which came first, the chicken or the egg? Those who believe in creationism rely on holy books, like the Bible, which specify that animals were created. Thus, the chicken came first. But, as Harold McGee has pointed out, the eggs of reptiles preceded by far the evolution of the first birds; consequently, “[e]ggs … are millions of years older than birds.” He added that ” Gallus domesticus, the chicken, more or less as we know it, is only 4 to 5 thousand years old, a latecomer even among the domesticated animals” (1984: 55).

McGee placed the ancestors of Gallus domesticus (as the Romans named it) in Southeast Asia or India. Maguelonne Toussaint-Samat was a bit more specific in writing that the chicken is “a descendant of a bird from the Malaysian jungle” (1992: 351). Still others have designated Burma and quite recently Thailand as its home-land, and there has been an argument for multiple origins with as many as four different species of jungle fowl contributing to the modern chicken (Smith and Daniel 1982; Fumihito et al.1994; Simoons 1994).

The chicken was not only a tardy arrival as a food animal; it may have been domesticated not for food purposes at all but rather because of a perceived need for an on-hand supply of birds for sacrifice and divination. Frederick Simoons (1994) pointed out that these have been traditional uses of the chicken in Southeast Asia and are still roles the bird is called upon to fill by many peoples of the region.

Another early, but nonfood, use for the chicken in Southeast Asia was the sport of cockfighting-a sport that to this day remains immensely popular there, despite thousands of years of opposition by religions such as Hinduism and Buddhism (Simoons 1994).

But cockfighting was not confined to Southeast Asia, and as interest in it spread, so did the chicken. Although there was mention of the bird in Egypt in the early fourteenth century B.C., there seems to be no subsequent mention of it for many centuries thereafter, prompting speculation that it may have disappeared after its initial introduction (Smith and Daniel 1982).The chicken, however, was also in Persia at an early date, and from there it (along with cockfighting) spread out into ancient Greece, where it joined ducks, geese, and guinea fowl in poultry yards around the fifth century B.C. (Toussaint-Samat 1992).

Although a few recipes from the Greeks indicate that eggs were used for baking, and the physicians who compiled the Hippocratic Corpus recommended lightly cooked eggs as the most nourishing way to prepare them, and Aristotle systematically opened eggs at different points in their incubation to describe what he saw, it is doubtful that egg production was, initially at least, a very important reason for maintaining the bird (Smith and Daniel 1982; Toussaint-Samat 1992).

In the Roman period, however, although cockfighting remained a primary reason for keeping chickens, eggs were finally beginning to catch on in the kitchen. The recipes of Apicius (25 B.C.) reveal custards, omelets, and eggs used in a variety of other dishes, as well as by themselves in hard-boiled form (McGee 1984; Touissant-Samat 1994). The Roman physician Galen, however, condemned fried eggs, saying they “have bad juice and corrupt even the foods mixed with them”(cited in Smith and Daniel 1982:366).

Yet in other parts of the world at about this time, it would appear that egg consumption was avoided more often than not. In part this was because eggs (as well as chickens) were used for divination, in part because eggs were regarded as a filthy food (the product of the semen of a cock), in part because of food taboos (such as those that regulated the diets of pregnant women and their youngsters), and often because it was believed wasteful to eat the egg instead of waiting for the chicken.

China, however, constituted a very large exception to egg avoidance. Both chickens and eggs were important sources of animal protein, and the Chinese are said to have encouraged their use throughout the rest of East Asia. In Southeast Asia and in the Pacific Islands (where the chicken was distributed early on by Asian colonizers), as well as in China, a taste was developed for brooded eggs (with well-developed fetuses). In addition, the Chinese became partial to “100-year-old” eggs that tend to make Westerners gag. But despite the name, the eggs are buried for only a few months in a mixture of saltpeter, clay, tea leaves, and other materials that cause the shells to turn black. The interiors of such eggs take on a “hardboiled” appearance with green veins running through them (Toussaint-Samat 1992; Simoons 1994).

Moving from the East back to the West after the fall of Rome, darkness descended on both Europe and the chicken, and little is known about the use of eggs on the Continent until the sixteenth century. In the meantime, the Iberians had discovered the New World, triggering many debates for scholars-among them the question of whether chickens were on hand in the Americas when Columbus first arrived in 1492. The answer seems to be no, at least as far as the West Indies (where the Spaniards introduced them) are concerned, but may very possibly be yes, as far as South America is concerned. There, the native people had names of their own for a kind of chicken-and the only one in the world that lays blue and green eggs (Smith and Daniel 1982). Page Smith and Charles Daniel have speculated that this South American araucana was not an “indigenous chicken” but instead the product of a union of American grouse with Asian chickens that reached South America with earlier, unrecorded voyagers (1982: 31), most likely Pacific Islanders.

European literature is not completely silent on eggs prior to the Renaissance; we are warned in Don Quixote de la Mancha (by Miguel de Cervantes [1547-1616]) not to put them all in one basket, and Francis Bacon (1561-1626) commented on those who would burn their houses down just to roast an egg. Throughout the preceding Middle Ages, however, eggs were mostly mentioned in nursery rhymes. In large part, one suspects that this considerable silence was because eggs were classified as meat by the Church; what with Fridays, Lent, and numerous other days when meat was proscribed, eggs were off the menu about half of the days of the year. As a consequence, most eggs were either used for hatching or were saved to be eaten at Easter. To keep them from spoiling, they were dipped in either liquid fat or wax and then decorated to make them more attractive-hence the custom of “Easter eggs” (Toussaint-Samat 1992).

Although some French and English recipe books from the late fourteenth century contain directions for making custards, omelets, and baked eggs (McGee 1984), Smith and Daniel (1982) suggest that the renaissance of the chicken came only in the sixteenth century with the work of Ulisse Aldrovandi (1522-1605), an Italian who wrote nine volumes on animals, including one on chickens. It was the dawn of a new age of science, and as Aristotle had done some 1,800 years earlier, Aldrovandi systematically examined the egg while at the same time adroitly dodging “that trite and thus otiose … question, whether the hen exists before the egg or vice versa” (quoted in Smith and Daniel 1982: 45). It might have been a new age of science, but the Church still had a firm answer for this old riddle.

The egg had made considerable culinary headway by 1651, when Pierre François de la Varenne published Le cuisinier françois, a cookbook that provided 60 recipes for eggs (Tannahill 1989). But it is the era embracing the eighteenth and early nineteenth centuries that has been characterized as “The Century of the Chicken” (Smith and Daniel 1982) because of the considerable amount of scientific interest the bird generated. Upon learning about elaborate hatching ovens in Egypt, the French naturalist René de Reaumur wrote a treatise on the subject, and breeding chickens became a preoccupation of European (and North American) country squires with a scientific turn of mind (McGee 1984).

This effort was given considerable impetus in the nineteenth century with the opening of the Chinese port of Canton (in 1834) to foreign traders. One of the first English vessels in the new trade returned with a few chickens of a Chinese breed-“Cochin” fowl, as they were ultimately called-as a present for Queen Victoria. In addition to their startlingly spectacular appearance, the Cochin chickens were superior in meat and egg production to established Mediterranean and European breeds. When they were first exhibited in England, tens of thousands of people showed up to stare, and all breeders had to have one. The same phenomenon took place at the Boston Poultry Show of 1849, where the birds again attracted crowds in the thousands, and even Daniel Webster attended to heap praise on the fowl.

Chickens had suddenly gained a new prominence among barnyard animals, and others of the great Asian breeds followed the Cochin to the West to perpetuate the chicken craze (Smith and Daniel 1982; McGee 1984). Breeding for show-with major emphasis on feather coloring and comb type-continued throughout much of the rest of the nineteenth century, with over 100 different breeds and color variations the result.

The “Century of the Chicken” was also a century of the egg, during which it was incorporated into diets as never before. Boiled eggs for breakfast became a favorite of many, and it was said that entire families of Parisians crowded around every Sunday to admire the dexterity of their sovereign, Louis XIV, who could knock off the small end of an egg with his fork in a single stroke (Toussaint-Samat 1992). The king was also very fond of meringues. Cookbooks provided careful instructions for the preparation of omelets and the poaching of eggs; mayonnaise was invented in the middle of the eighteenth century; the Americans followed the English example of marrying bacon with eggs; and the baking industry boomed (Trager 1995). Consequently, as the end of the nineteenth century approached, eggs were very much in demand in the West, and the emphasis in chicken breeding shifted from show-bird characteristics to productive capacity for eggs, or meat, or both.

Early in the twentieth century, Artemus Ward, in The Grocer’s Encyclopedia, indicated something of the way that demand was being met. He wrote of “large poultry farms [where] eggs are produced and handled very much as the product of any other factory … but,” he added, “the greater part of the country’s egg supply is still represented by accumulations from thousands of general farmers scattered all over the country” (1911: 223).

This is hardly the case today. Poultry sheds have become meat and egg factories with automated hatcheries (McGee 1984). Major steps in this direction took place in the 1930s and 1940s as John Tyson pioneered the vertical integration of the poultry industry. In 1956, the animal health-products division of Merck Pharmaceutical Company began production of a drug that prevented flock-destroying epidemics of coccidiosis. These events were accompanied by the development of high-protein feeds, after which chicken and egg production became truly automated industries (Trager 1995).

The egg is a fine (and cheap) source of high-quality protein, as well as iron, vitamins E and B12, folacin, riboflavin, and phosphorous, and was long regarded as a near-perfect food. But the egg’s yolk is also a source of considerable cholesterol, and with the implication of this substance in blocking heart arteries, demand for fresh eggs fell by almost 25 percent in the decade of the 1980s. In addition, eggs have been blamed of late (with some regularity) for outbreaks of salmonellosis. But although the use of fresh eggs has fallen off, the sale of food products containing eggs has risen significantly (Margen et al. 1992).The industry is hardly a dying one.

The Shell Egg Industry

The egg industry in most of the world is based on chicken eggs. In Southeast Asia there is also a duck egg market. As ethnic populations move, the food products they desire move with them. With the rather large number of Southeast Asian natives now living in other parts of the world, a geographically widespread demand for duck eggs has become a relatively recent phenomenon.

Table II.G.7.1, which includes only the markets for chicken eggs, lists the countries leading in egg production. Data on egg production in China, prior to 1989, are not available, but production there has always been extensive. Prior to 1940, China was the leading nation in the export of dried egg products. In Turkey and Korea, egg production has recently become significant. Figures for Eastern European countries for the 20-year period 1969 to 1989 show an increase in egg production in most of the countries, especially in the former Soviet Union. In countries with production controls, such as Canada, Australia, and the United Kingdom, there was almost no increase. The lower production in the United Kingdom for 1989 was likely influenced significantly by bad publicity relative to the safety of eggs during the mid-1980s. Overall, most countries listed in Table II.G.7.1 had an increase in egg production in excess of population increase.

The concentration of egg production has moved rapidly to larger units in the United States. In 1959, over 49 percent of all eggs in the United States were produced by flocks of less than 1,600 hens. By 1974, only 5.44 percent of all eggs were produced by such small flocks. In 1990, less than 1,000 companies produced over 97 percent of all eggs, with the smallest of these commercial farms having over 30,000 layers.

Egg consumption figures for countries around the world can be approximated by dividing human population figures for each country into the production figures listed in Table II.G.7.1.There is an international trade for eggs, but in most countries it is a relatively small percentage of total production. The Netherlands might be an exception, as they export many eggs to other European Economic Community (EEC) countries. Shell egg consumption in the United States has been declining since about 1950, when annual consumption was about 400 eggs per person with over 95 percent being in shell egg form. The 1990 consumption of shell eggs had decreased to about 200 per capita, with egg products accounting for over 40 more eggs per person. The use of egg products and eggs in prepared form has been increasing by 1 to 2 percent annually for the last several years. Publicity about potential danger from bacteria in shell eggs will likely result in a continuation of the shift to pasteurized egg products, particularly by institutional feeding establishments.

The egg products industry in the United States is expanding in numbers of companies involved and in volume from each company. The basic products are liquid, frozen, or dried whole egg, egg white, and egg yolk. For reasons of convenience, the shift has been from frozen and dried to liquid products. Developments in pasteurizing procedures and in the aseptic handling of liquids suggest that the shift to liquid will accelerate.

The egg industry is in a growth phase worldwide and will continue to expand because of the wide acceptance of eggs as food of a high nutritional value, especially in terms of protein quality, which makes the egg such a valuable food source for the expanding world population. Research and technology are striving to overcome the problems presented by cholesterol content and microbiological safety.

Egg Formation

At the time of hatching, the female chicken has a rudimentary ovary with over 2,000 ovules or immature egg yolks called oocytes. The egg yolk develops while in a follicle of the ovary. The remainder of the egg, especially the white, or albumen, and the shell, is formed in the oviduct. From these values it is apparent that major changes in the size of the reproductive system take place in relatively short time periods.

It is generally acknowledged that a hen forms an egg in about two weeks. This is true except for the very small core of the yolk. The yolk of the egg is formed in three stages: (1) the part formed during embryonic development of the female chick; (2) the normal slow development of the ovum from the time of hatching to a point in sexual maturity some 10 days prior to ovulation; and (3) the accelerated growth period during the last 10 days before ovulation (release of the ovum or yolk into the oviduct) (Stadelman and Cotterill 1995). The yolk increases in size during the rapid growth stage by the deposition of layers of yolk material. The concentric rings of growth remain intact until the yolk membrane, the vitelline membrane, is broken. The yolk of a normal chicken egg makes up 27 to 33 percent of the total egg weight.

In the oviduct the albumen, or white portion, of the egg, which accounts for about 60 percent of the total egg weight, is deposited around the yolk in about 4.5 hours. The oviduct is made up of five distinct regions. The yolk is released from the ovarian follicle into the infundibulum. A small amount of thick albumen rich in mucin fibers is deposited around the yolk during its 20-minute time of passage, due to peristolic action, into the magnum section of the oviduct. In the magnum section, the yolk collects the rest of the thick albumen and some thin albumen. The developing egg spends about 3 hours in the magnum section prior to entering the isthmus section where the shell membranes and remaining thin white are added during about a 1-hour stay.

The edible portion of the egg is now complete, but about 19 hours is spent in the uterus section where the shell is deposited over the shell membranes. In the posterior section of the uterus the pigments of colored egg shells are deposited on the shell just prior to movement of the intact egg through the vagina.

From the time the first thick albumen is deposited on the yolk until the shell membranes are in place, the yolk spins due to the peristolic movements of the oviduct. The spinning motion causes mucin fibers of the thick albumen to be twisted on opposite sides of the yolk into the chalaza. The chalaza forms a tight mesh over the yolk surface with the loose ends enmeshed in the layers of thick albumen. The chalaza aids in keeping the yolk centered in the albumen.

Another part of the egg often observed is the air cell. This forms between the inner and outer shell membranes after the egg is laid and is usually found in the large end. At the time of lay, the egg is at the body temperature of the hen, about 107° F (42° C), but as it cools, the volume shrinks, which starts the air cell. During the time the egg is held, there is a continual loss of moisture from the egg albumen, which results in an ever increasing air cell size. The rate at which moisture is lost is controlled by atmospheric humidity and porosity of the shell.

Egg Structure

As discussed in the section on the formation of the egg, the ovum starts development during the embryonic growth of the female chick. The female germ cell is in this structure. During the rapid growth phase of egg formation, the latebra grows to keep the germinal disk on the surface of the yolk. The vitelline membrane expands quickly during the rapid growth phase of yolk development. In a fully developed yolk the vitelline membrane is about 0.024 millimeter (mm) thick (Needham 1931). Depending on the methods used in histological examination of the membrane, there are either two or three layers (Romanoff and Romanoff 1949).

The yolk is deposited in concentric layers during formation. In hard-cooked egg yolks it is possible to differentiate light and dark rings when highly pigmented feedstuffs are fed for a limited period of time each day. The size of the yolk varies directly with egg size. The percentage of the total egg in the yolk varies from 27 to 33 percent. The percentage of the egg as yolk varies with egg size (Marion et al. 1964), with strains laying smaller eggs having a higher percentage of the egg as yolk. According to F. E. Cunningham, O. J. Cotterill, and E. M. Funk (1960), as hens age, the percentage of yolk in the egg increases. Cotterill and G. S. Geiger (1977) studied yield records from the Missouri Random Sample test and found that the yolk size in eggs decreased from 1965 to 1975.W.W. Marion and others (1964) suggested that all deviations in egg component part percentages are covariates of egg size.

The size of the yolk increases slowly during storage due to a very slow balancing of osmotic pressures between the yolk and albumen. A series of papers by J. Needham and M. Smith (1931), Smith and J. Shepherd (1931), Smith (1931), J. Needham, M. Stephenson, and D. M. Needham (1931), and J. Needham (1931) reported on the relations of yolk to albumen in the hen’s egg. The vitelline membrane is a unique entity in its ability to maintain about 50 percent solids in the yolk, with only 13 percent solids in the albumen over many months of storage. At this time, a proven explanation for this phenomenon is not available.

The albumen of the hen’s egg consists of four layers that can be identified in the broken-out freshly laid egg. The inner thick, or chalaziferous, layer is next to the vitelline membrane. This layer of albumen is rich in mucin fibers that are generally lacking in the thin white layers.The outer thick layer is a complex of lysozyme-mucin proteins. This complex disintegrates over time when eggs are in storage.

The shell membranes, inner and outer, are composed of keratin fibers arranged in a web pattern to form a barrier to microbial invasion of the egg contents. The inner membrane is about one-third the thickness of the outer membrane (Romanoff and Romanoff 1949). The combined thickness of the two membranes is about 0.1 mm. Passage of gases or liquids through the membranes occurs largely by osmosis or diffusion.

The egg shell must meet a number of requirements. It must be strong enough and rigid enough to withstand the weight of the adult hen. It must be porous enough to allow respiration for the developing embryo during incubation and still compact enough to prevent microbial invasion or the escape of too much moisture. Additionally, the shell serves as a supply of minerals for the nutrition of the embryo. The outer surface of the egg is the shell, which consists of four layers, all composed primarily of calcium carbonate. The thickness of the egg shell varies among hens. It is normally thickest for hens just starting egg production; thickness decreases as the hens extend their egg-laying cycle. A hot environmental temperature also results in thinner egg shells, as does a low level of calcium in the hen’s diet.

Egg Quality Evaluation and Preservation

The basis for quality determination of eggs by nondestructive means in the United States is the Egg Grading Manual (U.S. Department of Agriculture 1977). The same characteristics laid out in the manual are used in all countries with various degrees of emphasis on the several quality factors. Quality determinations are divided into external and internal factors. External quality factors are soundness of the shell, cleanliness, and egg shape. The internal factors are air cell size, albumen viscosity, and yolk shadow. The internal factors are judged by passing the egg in front of a bright light source. Other internal factors are freedom from blood spots, bloody albumen, meat spots, or other inclusions. Equipment is currently available that allows for quality evaluation by candling at rates in excess of 72,000 eggs per hour.

In 1981, the quality standards for grades of eggs in the United States were modified (U.S. Department of Agriculture 1981). The grades of eggs that can be offered for sale at retail are AA,A, B, and B*, referred to as B star.The requirements for each grade are detailed for each external and internal quality factor. The application of these standards are discussed by W. J. Stadelman and Cotterill (1995).

Numerous laboratory methods for quality evaluation of eggs have been prepared. Most of these methods are destructive in that the shell is broken and measurements are made on the liquid contents. The most widely accepted method is the Haugh Unit for expressing the albumen condition. This measurement was presented by R. R. Haugh (1937) and modified by A.W. Brant, A.W. Otte, and K. H. Norris (1951). A lesser used system is the United States Department of Agriculture (USDA) score as suggested by Brant and colleagues (1951), which attempts to correlate visual appearance of the broken-out egg, Haugh Units, and candled grade.

Evaluation of shell quality is on the basis of shell strength. The breaking strength has been found to be closely related to shell thickness. On broken-out eggs, measurement of shell thickness is a common method. As the specific gravity of freshly laid eggs is determined primarily by shell thickness, a nondestructive estimation of shell thickness can be made by determining specific gravity of the intact egg.

In terms of quality preservation, the most frequently considered item is albumen condition, which is often expressed as Haugh Units. For a high-quality or grade AA egg, the Haugh Units should be above 78. This value lowers over time as the thick albumen thins. The rate of albumen thinning is a function of temperature. The breakdown or thinning of albumen is relatively rapid at high temperatures, 40° Celsius (C), and slows to almost no change at 1° C. Other than temperature, the carbon dioxide content of the atmosphere surrounding the egg affects the rate of carbon dioxide loss from the egg. With the loss of carbon dioxide, the pH of the albumen rises from about 7.6 in a fresh egg to 9.7 in a stale egg.

Humidity of the atmosphere influences rate of water loss from the egg, which results in increased air cell size. Ideal conditions for long-term storage of eggs are a temperature between 1° C and 3° C and a relative humidity of about 80 percent. For long-term storage, egg shells are usually coated with a colorless, odorless mineral oil that seals the pores of the shell.

A frequently neglected consideration in quality preservation is maintaining cleanliness and soundness of the shells. In summary, egg quality preservation is a function of time, temperature, humidity, and handling.

Egg Sizes

In most parts of the world egg sizes are based on metric weights, with each size up or down being with a 5-gram grouping. Eggs are sold by the dozen in the United States. Egg sizes are based on weight per dozen ranging from pee wee (15 ounces [oz]), small (18 oz), medium (21 oz), large (24 oz), extra large (27 oz), to jumbo (30 oz). For consumer satisfaction it is desirable to pack cartons holding a dozen with uniform-sized eggs of the appropriate weight. Many of the jumbo-sized eggs will contain two yolks.

Egg Chemistry

Shell eggs consist of about 9.5 percent shell, 63 percent albumen, and 27.5 percent yolk, according to Cotterill and Geiger (1977). The total solids of the albumen, yolk, and whole egg are about 12 percent, 52 percent, and 24 percent, respectively. Reviews of the chemistry of egg components were written by R. E. Feeney (1964), T. L. Parkinson (1966), Stadelman (1976), and W. D. Powrie and S. Nakai (1990). Extensive analytical data on the nutrient composition of eggs are given in a publication by the U.S. Department of Agriculture (1989).

The shell membranes are composed of protein fibers, mostly keratin with some mucin, arranged in net fashion to form a semipermeable membrane. The shell membranes provide a significant barrier to bacterial invasion of the albumen.

The egg white consists of a number of proteins, a small amount (less than 1 percent) of carbohydrates and minerals, and no lipids. The composition of the albumen of a freshly laid egg is about 87 percent moisture, 11.5 percent protein, 0.8 percent ash, and 0.7 percent carbohydrates. The carbohydrates are the sugars in glycoproteins. The predominant protein is ovalbumin. The complete amino acid sequence with 385 residues has been determined by A. D. Nisbet and others (1981). The molecular weight of the polypeptide chain is 42,699.

The pH of the albumen in a freshly laid egg is about 7.6, which increases during storage to as high as pH 9.7. The rate of pH change is influenced by temperature, air movement, and shell quality. The pH increase is the result of carbon dioxide loss from the albumen. The pH of the albumen depends on the equilibrium between the dissolved carbon dioxide, bicarbonate ion, carbonate ion, and proteins.

The protein ovotransferrin is referred to as conalbumin in older literature. Its unique characteristic is its ability to bind multivalent metallic ions. With aluminum salts a white precipitate is formed; with copper, the flocculant is yellow; and with ferric iron a red color results. The ovotransferrin is one of the most heat-labile proteins of the albumen. Powrie and Nakai (1990) discuss methods for the isolation of each of the proteins.

Egg yolk might be described as a complex system containing a variety of particles suspended in a protein solution.Another description is that it is an oil-in-water emulsion, with lecithin and the lysoproteins aiding in maintaining a stable emulsion. The yolk consists of a plasma with suspended granules. Fresh egg yolk contains about 52 percent solids and has a pH of 6.0, which rises during storage to 6.9.

Nutritional Value

Eggs are a popular food in all countries of the world and have been since ancient times. Before agriculture developed, eggs were gathered from birds’ nests for human food. Although eggs contain about 75 percent water, they are a rich source of high-quality protein and are often used as the protein against which other protein sources are compared. Eggs are also important sources of unsaturated fatty acids, iron, phosphorus, trace minerals, vitamins A, E, and K, and all B vitamins. As a natural source of vitamin D, eggs rank second only to fish oils. Eggs are low in calcium, as the shell is not eaten, and they are devoid of vitamin C (Stadelman and Pratt 1989). The high nutrient density of eggs relative to their caloric content makes them an excellent food for many people with special dietary needs (Stadelman et al. 1988).

Much has been written concerning the relationship of plasma cholesterol level to coronary problems in humans, yet some individuals do not adequately differentiate between plasma, or serum, cholesterol and dietary cholesterol. For most people it would seem that there is only a very slight relationship between the level of cholesterol in their diet and the serum cholesterol level in their blood.

Microbiology

The egg contents at the time of laying are generally free of microbial contamination. During the 1980s it was found that a few eggs, estimated at 1 in 20,000, might contain a bacteria, Salmonella enteriditis, in the yolk at the time of production. This bacteria is of great concern to health officials and the egg-producing industry because the organism can cause food poisoning in humans.

The egg has a number of barriers to bacterial invasion. The shell and shell membranes act as physical barriers. In the albumen there are several proteins that influence the ability of organisms to colonize in the egg. Lysozyme will digest cell walls of some bacteria. Avidin removes biotin from the available nutrients for bacteria, and ovotransferrin chelates with ions of iron, making it unavailable. When eggs are cooked, the tying up of biotin or iron is eliminated so these materials are readily available for microorganisms or humans.

Packaging

As egg shells are subject to cracking, packaging has been developed to minimize this loss. In early days eggs were loosely packed in rigid containers using whole grain oats to keep the eggs from contacting the sides of the container or each other. A step forward in innovation was the use of fillers and flats made of cardboard to keep eggs in individual cells in wooden cases. The wooden case was standardized to hold 30 dozen eggs. The next move was to fiberboard cases and a pulp paper filler flat. These cases are now in use for domestic shipments. Most export shipments are still in wooden cases using the filler flat.

For the retailing of eggs, many packages have been and are still being used. The poorest of these, as far as protecting the egg, is a bulk display with paper bags to carry a dozen or more eggs. Pulp paper cartons have been developed to hold from 2 to 18 eggs. These may be coming back into use because of environmental concerns regarding plastic foam cartons.

The Liquid Egg Industry

Shell eggs are converted to liquid products by the removal of the shell. It is required in the United States and some other countries that the shells be removed with no commingling of the shell and liquid portions. The liquid content of the egg is handled as albumen, yolk, whole egg, and various blends of yolk and albumen. Commercial equipment can break and separate yolks from albumen at rates in excess of 36,000 eggs per hour.

Processing Liquid Eggs

The steps of processing liquid eggs include pasteurization, homogenization, packaging, and refrigeration. The liquid products may be converted to frozen, dehydrated, or dried products. An excellent historical review covering early development of the United States egg-products industry was prepared by J. W. Koudele and E. C. Heinsohn (1960).

The pasteurization of eggs is accomplished by heating the liquid to a sufficiently high temperature for a long enough time to kill any pathogenic microorganisms that might be present. As egg proteins coagulate when heated, the pasteurization temperatures and times must be carefully controlled in order to obtain the bacterial reduction with minimal damage to the functional properties of the egg product. Heat damage can be minimized by maintaining a high degree of turbulence in the egg product during the pasteurization process. For whole eggs in the United States, a temperature of 60° C (140° F) for a minimum of 3.5 minutes is required. Higher temperatures for a shorter holding time are accepted. C. R. Murdock and others (1960) compiled information on minimal accepted pasteurization times and temperatures in several countries. In Poland whole egg pasteurization requires 66.1° C to 67.8° C for 3 minutes, in China 63.3° C for 2.5 minutes and in Australia 62.5° C for 2.5 minutes.

Pasteurization temperatures for albumen are sometimes lower than for the whole egg, using longer holding times.The addition of aluminum salts to the albumen results in chelation, with the ovotransferrin allowing the use of higher pasteurization temperatures for albumen (Lineweaver and Cunningham 1966). Ovotransferrin is the most heat-labile of all proteins in the albumen. Egg yolk is rather heat-stable and can be pasteurized by using slightly higher temperatures or longer times than for the whole egg. Cunningham (1990) reported detailed procedures for the pasteurization of different liquid egg products.

A procedure for the ultrapasteurizing of liquid whole eggs was described by H. R. Ball, Jr., and colleagues (1987), and by K. R. Swartzel, Ball, and M. Hamid-Samimi (1989). They utilized temperatures that are higher than the minimum required in the United States with high turbulence in the holding tubes and projected a usable shelf life of the ultrapasteurized whole egg products of up to 24 weeks when stored at 4° C or lower. The commercialization of this procedure will reduce the need for frozen products.

The homogenization of liquid whole egg may be done either before or after pasteurization. It usually follows pasteurization so that any clumping of materials due to the heating would be dispersed in the homogenization operation.

The packaging of egg products is an ever changing operation. The early processors, from the late nineteenth century until the mid-twentieth century, used metal cans, each containing about 30 pounds of product. The next package was a rigid plastic bucket of either 10- or 30-pound capacity. This was followed by the use of waxed cardboard boxes of 4-or 5-pound capacity. Small quantities were packaged in half-pint cardboard containers in a futile attempt at selling the egg products in retail food stores. At present the industry is moving toward the use of flexible film packages with an aseptic packaging technology.

Before the introduction of ultrapasteurization and aseptic packaging, the shelf life of liquid eggs was about 6 days. But now, with a predicted shelf life of up to 24 weeks (Swartzel et al. 1989), it may be possible to sell liquid product at retail. In earlier years much of the liquid product was packaged as indicated and frozen for distribution to food manufacturers.

During World War II there was a great demand for dried egg products for military feeding and to supply civilian populations of countries in the war zones. The usual method of drying was by using a spray drier. Small amounts were pan-dried and freeze-dried. In the drying of eggs it was necessary to remove the reducing sugars normally present in egg albumen to minimize the discoloration that would result from a reaction between such sugars and amino acids abundant in the egg liquid. Desugaring is accomplished by a controlled bacterial or yeast fermentation, by a spontaneous microbial fermentation, or by an enzymatic fermentation using glucose oxidase.

During the last 20 years a number of additional processed egg products have been introduced in the marketplace, as described by Stadelman and others (1988).The most widely marketed to date have been hard-cooked egg products. They are available as peeled hard-cooked eggs, diced eggs, long eggs, and Scotch eggs. It was estimated that about 1 percent of all eggs sold in the United States in 1990 were in hard-cooked form. Generally, the hard-cooked egg sales are to restaurants rather than at retail.

In summary, eggs have long served as a food for humankind. They have a high-density nutritional value, especially with respect to the protein quality. They contain all nutrients required by humans except vitamin C, but as we do not utilize the shell, the egg is also deficient in calcium. Eggs are usually one of the first solid foods given to infants and frequently constitute a significant portion of the diet of the elderly. Valued for their versatility, they are employed to coagulate, to foam, to emulsify, to color, to flavor, and to control sugar crystallization in some candies. With this wide range of use, eggs remain one of our most economical food staples.